In recent years, coating an inner surface of, for example, pet (PET) bottle that is one of plastic containers with a hard carbon film such as DLC (diamond like carbon) has been tried to prevent oxygen from permeating from the outside and carbon dioxide from permeating from the inside (for example, carbonated drinks), and various apparatus for film formation therefor have been offered (see Patent Documents 1 to 3).
Here, as an apparatus for film formation to form a carbon film on a plastic container by making use of high frequency plasma CVD (chemical-vapor deposition) method, an explanation of the apparatus according to Patent Document 3 that is a basic invention to coat the inside of a container is given with reference to FIG. 9.
As shown in FIG. 9, the apparatus for film formation is an apparatus to carry out film formation by discharge plasma on the inner surface of a plastic container 12 having an opening 11, and the apparatus is provided with an outer electrode 13 composed of an outer upper electrode 13-1 and an outer lower electrode 13-2 that are large enough to enclose the outer periphery of the plastic container 12, an exhaust pipe 14 attached via an insulating member 26 to an end face of the outer electrode 13 on the side where the opening 11 is arranged, an inner electrode 17 that is inserted from the exhaust pipe 14 side into the inside of the plastic container 12 in the outer electrode 13 approximately over the full longitudinal length of the plastic container 12 and is connected to the ground side as well as pierced with a gas outlet 16 to blow-off a medium gas G, an exhauster that is not shown but attached to the exhaust pipe 14, a gas supply unit that is not shown but supplies the medium gas G to the inner electrode 17, and a high frequency power source 18 connected to the outer electrode 13.
Here, the outer electrode 13 is provided inside a cylindrical chamber 22 having flanges 21a and 21b at the upper and lower ends, respectively, and the cylindrical chamber 22 is placed and held on an annular base 23. A circular insulating plate 24 is arranged between the annular base 23 and the bottom back side of the outer lower electrode 13-2. A cylindrical insulating member 20 is provided at the end of the gas outlet 16 of the inner electrode 17, so that local plasma concentration is prevented.
A method of coating a bottle with a carbon film with the use of an apparatus in such a structure is explained.
First, the plastic container 12 is inserted inside the outer electrode 13, and inner gas is exhausted through the gas exhaust pipe 14. After having reached a predetermined vacuum (representative value: 10−1 to 10−5 Torr), the medium gas G is supplied to the inner electrode 17 at a flow rate of, for example, from 10 to 200 mL/min, and further blown off to the inside of the plastic container 12 through the gas outlet 16 of the inner electrode 17. Note that as this medium gas G, for example, benzene, toluene, xylene, aliphatic hydrocarbons such as cyclohexane, aromatic hydrocarbons, oxygen-containing hydrocarbons, or nitrogen-containing hydrocarbons are used. The pressure inside the plastic container 12 is set to, for example, from 2×10−1 to 1×10−2 Torr depending on the balance between a volume of gas supply and a volume of exhaust. Then, high frequency power of from 50 to 2000 W is applied from the high frequency power source 18 through an impedance matching unit 36 and a RF (radio frequency) input terminal 35 to the outer electrode 13.
By the application of high frequency power to the outer electrode 13 in this way, plasma is generated between the outer electrode 13 and the inner electrode 17. At this time, since the plastic container 12 is housed inside the outer electrode 13 with almost no space, plasma is generated inside the plastic container 12. The medium gas G is dissociated by the plasma or further ionized to generate film forming species to form a carbon film. The film forming species deposit on the inner surface of the container 12 to form a carbon film. After the carbon film is formed to have a predetermined film thickness, the application of high frequency power is stopped, the medium gas supply is stopped, the residual gas is exhausted, nitrogen, rare gas, air, or the like is supplied inside the outer electrode 13, and the pressure in this space is returned to atmospheric pressure. After this, the plastic container 12 is removed from the outer electrode 13. In this method, it takes two to three seconds to form a carbon film having a thickness of 20 to 30 nm.
Patent Document 1: Patent Application Laid-Open No. H8-53116
Patent Document 2: Japanese Patent No. 2788412
Patent Document 3: Patent Application Laid-Open No. 2003-286571